A bat detector
is a device which picks up the ultrasonic echolocation and social calls
of bats and converts them into the human audible range. They work on
three different principles:

Heterodyne:-This type of bat detector
is similar in operation to and old fashioned radio with a tuning control
knob, where you turn the knob to your required radio station, you can only
listen to one station at a time. This is because the radio receiver has
a very narrow receiving bandwidth which only accepts one station.

The bat detector
operates in a similar fashion but instead of tuning over a range of radio
frequencies, it tunes over a range of ultrasonic frequencies. Hence you
may miss some bats calls if you are not at the correct tuned frequency.
The detector sound output is usually a series of clicks, ticks and tocks
and for some species a lovely warbling sound.

For
the Techno- Anaraks :-The heterodyne technique is to beat the
incoming bat frequency picked up by an ultrasonic microphone, with a local
tuneable oscillator. This is done using a mixer. The output of the mixer
contains the original ultrasonic frequency, the local oscillator frequency,
and the sum and differences frequencies of both oscillator and ultrasonic.
A low pass filter only allows the wanted sum and differences frequencies,
usually restricted to ± 5kHz, which is then amplified and fed to
a loudspeaker. This type of sound is completely artificial and does not
bear much relationship to the actual bat call other that call repetition
rate.

A Super Heterodyne
receiver does the mixer trick twice and utilises an Intermediate Frequency
( IF) amplifier, before the final ± 5kHz output drive.

Frequency
Division :-Out
of fashion nowadays. For these detectors the ultrasonic bat call is divided
down in frequency to cover human hearing range. On some detectors the amplitude
variations of the ultrasonic waveform are transferred to the divided down
frequency. These are wide band detectors capable of looking at the whole
of the ultrasonic frequency range at once, and as such do not require a
tuning control.

For
the Techno-Anoraks:-
Ultrasonic bat call is fed to a zero crossing detector. The output of
this is a digital signal which is then fed to a binary counter. Favourite
binary counter output used ÷32, but ÷16, and ÷ 64
can also be used. For amplitude transfer the ultrasonic bat call is demodulated,
and this signal is used to re-modulate the digital counter output. Hence
the amplified variation can be transferred to the divided down output.

Time Expansion:-This
is the new cool kid on the block. It is wide band in operation and receives
the whole of the ultrasonic range that is used by bats. No tuning is required.
It records all bat calls at all frequencies then plays back at a much lower
rate. This means that the ultrasonic frequency range is converted to an
audible human hearing range. When using the detector on location the slowed
down signal, referred to as a "time expansion" signal can be recorded onto
a tape recorder. The tape recorder output can then be fed to a sound card
in a PC for signal analysis, giving a very good tool for bat species identification.
Of course, the PC must have suitable software. Using this method an objective
assessment is available for determining bat species not subjective as in
the past using heterodyne detectors. The sound of a time expanded signal
is similar to the sound of whale calls, (whereas the whale call is speeded
up and not slower down.) All the nuances of the bat calls can be heard.

For
the Techno-Anaraks:-
Signals are slowed down by ten for PC work and thirty two for human ears.
Analogue to Digital sample rate typically 400 kHz to conform to the Nyquist
sampling criteria, since signals up to about 200kHz required to be recorded.
Eight bit resolution is generally considered as adequate. Length of ultrasonic
recording up to 20 seconds, but typically only 1 second needed. An average
bat calls last for 10 milli seconds usually an FM chirp but can also be
a CF repeated tone.•